Fixed bug in copper tutorial.

In the first example lines of the copper tutorial, the line
'view(cluster)' raised an error since the 'cluster' variable
was not yet declared. Replaced by 'view(copper)'.

doc/source/tutorials/copper/tuto_ped_copper.rst

@@ -49,7 +49,7 @@ This is the job of the *ase* module, so load the module
 
 	# Create the copper cubic cell
 	copper = bulk('Cu', a=a0, cubic=True)
-        view(cluster)
+        view(copper)
 
 In line 6 we load the :py:func:`bulk` function to create our atomic object and,
 in line 7, we load the :py:func:`view` function to actually view our cluster.